This problem, although seems to be complicated from the first glance, is actually very simple one unknown problem. Let x be the amount invested at 5%. Then you have this "total interest" equation 0.05x + 0.06*(0.25x) + 0.07*(80000 - x - 0.25x) = 4700. This equations tells everything about itself, you only need to hear it . . . I mean "to read it" . . . Please recognize that 0.25x is. Your job now is to simplify and solve it for x.